Transition Metal-Mediated DNA Adsorption on Polydopamine Nanoparticles

Polydopamine (PDA) is a widely used universal coating for a broad range of materials. Interfacing PDA with various biomolecules, such as DNA, is critical for applications such as sensing, intracellular delivery, and material fabrication. Because of the negative surface charge of PDA at neutral pH, electrostatic repulsion exists between PDA and DNA. In previous studies, modified DNA or low pH was used to overcome this repulsion for DNA adsorption. More recently, divalent Ca2+ was found to bridge DNA and PDA. Herein, we studied four transition metals (Mn2+, Co2+, Zn2+, and Ni2+) and compared their efficiencies with Ca2+ for promoting DNA adsorption. These transition metals induced a more efficient and tighter DNA binding compared to Ca2+. In all these cases, the DNA phosphate backbone played a dominant role in adsorption, although DNA bases might also interact with strong binding metals such as Ni2+. Moreover, when the adsorption affinity was stronger, sensing was more selective to complementary DNA. Finally, aging of PDA appeared to be detrimental for DNA adsorption, which could be due to further oxidation of PDA. We showed that using Zn2+ or Ni2+ could considerably relieve the aging effect, while storing PDA at 4 °C could slow down aging.